There are now four functions and one global constant:
* opal_progress_thread_name: the name of the OPAL-wide async progress
thread. If you have general purpose events that you need to run in
*a* progress thread, but not a *dedicated* progress thread, use this
name in the functions below to glom your events on to the general
OPAL-wide async progress thread.
* opal_progress_thread_init(): return an event base corresponding to a
progress thread of the specified name (a progress thread will be
created for that name if it does not already exist).
* opal_progress_thread_finalize(): decrement the refcount on the
passed progress thread name. If the refcount is 0, stop the thread
and destroy the event base.
* opal_progress_thread_pause(): stop processing events on the event
base corresponding to the progress thread name, but do not destroy
the event base.
* opal_progess_thread_resume(): resume processing events on the event
base corresponding to a previously-paused progress thread name.
